Biomphalaria tenagophila is a simultaneous hermaphrodite freshwater snail, which can breed by self and cross-fertilization. It is, therefore, well suited for studying reproduction as an evolutive strategy. Several characteristics (shell size and age at sexual maturity, egg fecundity and fertility) were analyzed in four consecutive self-fertilized generations and compared to cross-fertilized individuals. The reproductive parameters were similar in the two groups. Shell size was significantly greater in the fourth self-fertilized generation than in the cross-fertilized individuals.

INTRODUCTION

Self-fertilization is a reproductive mechanism that leads to a loss of genetic variability and affects the adaptive capacity of a species by altering the number of homozygous loci.

Pulmonate snails of the genus Biomphalaria can self-fertilize since they are simultaneous hermaphrodites (Paraense, 1955). Analysis of reproduction in freshwater hermaphrodite snails using genetic markers such as isozymes (Bandoni et al., 1995, for review) and microsattelites (Viard et al., 1996) shows heterozygote deficiencies, indicating that selfing could be a regular mating strategy.

What would then be the evolutive meaning of selfing? According to McCracken and Selander (1980), selfing could be a protection mechanism for genetic material accumulated along the evolutive history of pulmonate hermaphrodite snails. Biomphalariatenagophila specimens were submitted to four consecutive self-fertilizations in order to measure the efficiency of selfing under laboratory conditions.

MATERIAL AND METHODS

Biomphalaria tenagophila, a species commonly found from southeastern Brazil to northern Argentina (Paraense, 1981), was raised under laboratory conditions for five years, in stock aquaria containing 20 l of water, receiving lettuce ad libitum. A pool of capsules with many eggs was obtained from melanic snails from the stock population.

After hatching, 30 animals were chosen at random and kept isolated in mini-aquaria, where they reproduced by self-fertilization exclusively. This group was called SF1 (self-fertilized group 1). SF2 to SF4 snails were obtained in succeeding generations.

The following characteristics were observed for each of the 30 SF4 individuals:

1. shell size at sexual maturity (in mm); 2. age at sexual maturity (in days); 3. fecundity - expressed as number of eggs/egg capsule. Egg capsule number was obtained by collecting up to 100 eggs from each snail; 4. fertility - expressed as number of hatched eggs/number of eggs.

Simultaneously, the same variables were observed in a control group (CF) consisting of 30 albino snails previously fertilized by melanic snails of the same species. The means of each variable were compared by a one-way ANOVA (Neter and Wasserman, 1974).

RESULTS AND DISCUSSION

The only variable that differed significantly was size at sexual maturity (Table I). The means of the 4th self-fertilization generation were significantly greater than in the control group. On the other hand, age, fecundity and fertility showed no significant change after consecutive self-fertilizations.

The difference in size at sexual maturity shows that it is possible to alter phenotypic variability through self-fertilization, and contrary to the observations by Richards and Merritt (1975), shell size at maturity tends to be larger in self progeny than in cross fertilized ones. When comparing age and size at first sexual maturity, the results suggest that there was an acceleration in growth as a result of successive self-fertilizations. Nevertheless, the increase in mean population size in self-fertilization did not occur at the expense of a reduction in reproductive potencial, since SF4 fecundity and fertility were statistically similar to the control.

Self-fertilization had no negative effects on development and survival of B. tenagophila. This is somewhat surprising as, according to Charlesworth and Charlesworth (1990), self-fertilization leads to homozygosity. We conclude that lack of cohabitation with other snails is not a limiting factor to the reproductive potential of self-fertilized generations of B. tenagophila, contrary to the observations by Vernon (1995) that reproductive variables in Basommatophora are modulated by social facilitation.

B. tenagophila lives in an environment subject to radical changes (such as parasitism, moluscicides and drought), resulting in the loss of a large number of individuals (Tuan, 1996). In such instances, self-fertilization would be an effective mechanism for the maintenance of environmentally adapted genotypes, and therefore a mechanism for evolutional preservation.

ACKNOWLEDGMENTS

The authors are thankful to M.C.G. Porto Alves for statistical support. Publication supported by FAPESP.